[LLVMdev] Does getelementptr get optimized in compile-time?

[Journeyer J. Joh]

Hello Duncan Sands,

> If however the operands are all numerical values (eg the pointer operand is "null" and the others are numbers like "1") then it will be turned into a number by the optimizers.

I think my case is stated above among your answer.

http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt

There is a brief document about using getelmentptr trick above.
And I was wondering something similar to this case.

  %Size = getelementptr %T* null, int 1
  %SizeI = cast %T* %Size to uint

I understood that for this case, getelementptr will be collapsed to a
number during optimization. It is also explained in the brief document
above. I didn't understood by the document at that time.

OK. Thank you very much. I understood very well.

Sincerely
Journeyer


2013/1/3 Duncan Sands <baldrick at free.fr>:
> Hi Journeyer,
>
>
>> I've got a question about getelementptr.
>>
>>
>> I've been told that getelementptr is evaluated in compile-time NOT in
>> run-time.
>
>
> this is impossible if the arguments are not constants.  If they are
> constant,
> then the optimizers will turn a getelementptr instruction into a
> getelementptr
> constant expression
> (http://llvm.org/docs/LangRef.html#constant-expressions).
> Even so it usually isn't evaluable at compile time, since typically the
> pointer
> operand is a global variable for which the address is only known when the
> program loader loads the final executable into memory.  If however the
> operands
> are all numerical values (eg the pointer operand is "null" and the others
> are
> numbers like "1") then it will be turned into a number by the optimizers.
>
> Ciao, Duncan.
>
>
>>
>> So I tested it with clang web compiler below.
>> http://llvm.org/demo/index.cgi
>>
>> But with LTO nor Standard, neither one optimize getelementptr out.
>>
>> Does getelementptr get evaluated in compile-time?
>> Or doesn't it get executed in run-time? (I believed so. Now I am
>> wondering.)
>>
>> Thank you in advance.
>>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev



-- 
----------------------------------------
Journeyer J. Joh
o o s a p r o g r a m m e r
a t
g m a i l  d o t  c o m
----------------------------------------